1. Field of the Invention
The present invention relates to an image forming method and an electrophotographic image forming apparatus such as printers, reproduction devices, and facsimiles.
2. Description of the Related Art
Recently, with respect to image forming apparatuses such as printers, copiers, and facsimiles, the market demand of energy saving and speeding up is growing stronger. To obtain these performances, it is important to achieve improvement in heat efficiency of a fixing unit for use in an image forming apparatus.
In an image forming apparatus, by image forming processes such as electrophotographic recording, electrostatic recording, or magnetic recording, a toner image not having been fixed is formed on a recording medium such as recording sheets, printing papers, photosensitive papers, electrostatic recording papers by an image transfer method or a direct method. In fixing units for fixing such a toner image not having been fixed, contact heating systems such as heating roller system, film heating system, and electromagnetic induction heating system are widely employed.
The fixing unit of heating roller system has the basic construction of a pair of rotating rollers of a fixing roller that includes a heat source such as halogen lamp to be adjusted at a predetermined temperature, and a pressure roller that is pressed in contact with this fixing roller. A recording medium is inserted into the contact portion (the so-called nip portion) between a pair of these rollers, and delivered. Thus, a toner image not having been fixed is melted and fixed due to heat and pressure provided from the fixing roller and the pressure roller.
The fixing units of film heating system are proposed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 63-313182, and Japanese Patent Application Laid-Open (JP-A) No. 01-263679.
In such fixing units of film heating system, a recording medium is brought in close contact with a heating element that is fixed and supported by a support member via a thin fixing film having a heat resistance, and heat of the heating element is applied to the recording medium via the film member while the fixing film is made to slide and move with respect to the heating element.
As the heating element, for example, used is a ceramic heater in which there is provided a resistance layer on a ceramic substrate such as alumina or aluminum nitride having properties of heat resistance, insulating property, and high heat conductivity.
In this fixing unit, a thin fixing film of low heat capacity can be used, so that a higher efficiency of heat transfer is achieved than the heating roller system, and a shorter warm-up time period is achieved. Thus, more quick start or energy saving comes to be possible.
As the fixing units of electromagnetic induction heating system, proposed is art in which, for example, Joule heat is generated by an eddy current generated at a magnetic metal member with an AC magnetic field, and a heating element including a metal member is made to generate heat by electromagnetic induction (see Japanese Patent Application Laid-Open (JP-A) No. 08-22206).
The configuration of the fixing unit of electromagnetic induction system will be described. FIG. 1 is a schematic view showing a conventional fixing unit of an electromagnetic induction heating system. This fixing unit is constructed of a film inner surface guide 121 on which mounted is a heating element 120 formed of an exciting coil unit 118 and a magnetic metal member 119, being a heating part; a cylindrical film 117 having a heat resistance that contains therein the film inner-surface guide 121 in the state in which the magnetic metal member 119 is in contact with the inner wall; and a pressure roller 122 that is pressed against the film 117 in a position of the magnetic metal member 119 to form a nip portion N with this film 117, as well as that causes this film 117 to rotate.
As the film 117, used is a single-layer film such as PTFE, PFA, or FEP whose thickness is 100 μm or less, and which has a heat resistance, or a composite-layer film in which coating of e.g., PTFE, PFA, and FEP is made on the perimeter surface of e.g., polyimide, polyamideimide, PEEK, PES and PPS.
The film inner-surface guide 121 is formed of materials having rigidity and heat resistance that are made of resin such as PEEK or PPS. The heating element 120 is fitted substantially at the central portion in a longitudinal direction of such a film inner-surface guide 121.
The pressure roller 122 is formed of a core 122a, and a heat resistant rubber layer 122b having a superior releasing property such as silicone rubber that is located around the core 122a. This pressure roller 122 is located so as to be pressed in contact with the magnetic metal member 119 of the heating element 120 with the film 117 sandwiched under a predetermined compressive force provided by bearings or biasing unit (either one is not shown). Further, the pressure roller 122 is driven to rotate in a counterclockwise direction by driving unit (not shown).
Due to that the pressure roller 122 is driven to rotate, a friction is generated between the pressure roller 122 and the film 117, the rotary force acts on the film 117, and the film 117 is slidingly rotated while being in contact with the magnetic metal member 119 of the heating element 120.
In the state in which the heating element 120 has reached a predetermined temperature, a recording medium 111 including a toner image T not having been fixed that is formed at the image-forming section (not shown) is inserted into the nip portion N between the film 117 and the pressure roller 122. Owing to that this recording medium 111 is delivered at the nip portion N with sandwiched between the pressure roller 122 and the film 117, heat of the magnetic metal member 119 is applied to the recording medium 111 via the film 117, and a toner image T not having been fixed is melted and fixed on the recording medium 111. Moreover, at the outlet of the nip portion N, the recording medium 111 having been passed is separated from the surface of the film 117 to be delivered to a paper ejection tray (not shown).
In such a fixing unit of electromagnetic induction heating system, owing to the use of generation of an eddy current, the magnetic metal member 119 acting as induction heating unit can be located in the vicinity of a toner image T of the recording medium 111 via the film 117, resulting in further improved heating efficiency as compared with the foregoing fixing unit of film heating system.
However, fixing units in a full-color image forming apparatus need to be capable of sufficiently heating and melting a thick toner particle layer, being a laminate of four layers or more. To meet this need, in the fixing units of electromagnetic induction heating system, a rubber elastic layer of a certain thickness is required at the film surface to fully cover and uniformly heat and melt a toner image. In the case where the film surface is covered with an elastic layer such as silicone rubber to a certain extent, a thermal responsiveness becomes worse due to a low heat conductivity of the elastic layer, and thus there will be a significantly large temperature difference between the inner surface of a film heated from a heating element, and the film outer surface in contact with toner. In the case of a large amount of toner, the belt surface temperature is rapidly decreased, and a sufficient fixing performance cannot be achieved, and thus a problem exists in that the so-called cold offset occurs.
Meanwhile, in the image forming method, a latent electric image or a latent magnetic image is developed with toner. For example, in the electro-photographic image forming method, an electrostatic image (latent image) is formed on a photoconductor (hereinafter, may be referred to as “latent electrostatic image bearing member”, “image bearing member”, or “electrophotographic photoconductor”), this latent is developed using toner, and a toner image is formed. Subsequently, a toner image is normally transferred onto a recording medium such as paper, and fixed.
Toner for use in development is generally a colored particle in which a binder resin is included with a colorant, a charge control agent, and other additives therein. Manufacturing methods thereof are roughly divided into pulverization and suspension polymerization. In the pulverization, a colorant, a charge control agent, an anti-offset agent and the like are melted and mixed in a thermoplastic resin to be uniformly dispersed, and a toner composition having been obtained is pulverized and classified, whereby toner is manufactured. By this pulverization method, toner having a superior property to some extent can be manufactured, but selection of toner materials is limited. For example, a toner composition obtained by being melted and mixed has to be pulverized and classified using an apparatus to be economically usable. With this need, a toner composition melted and mixed cannot help being sufficiently fragile.
Therefore, practically, when the toner composition is pulverized into particles, it is likely that the particle size distribution thereof will be broad. Further, to obtain a copy image having high resolution and many levels of gray, fine powders of a particle size of 5 μm or less and coarse grains of a particle size of 20 μm or more have to be removed by classification, resulting in the disadvantage of extremely lower yield. Furthermore, in the milling, additives such as colorant and charge control agent are hard to be uniformly dispersed in a thermoplastic resin. Such a non-uniform dispersion of additives adversely affects a flow property, developability, durability, an image quality and the like. In addition, a problem exits in that there is restriction on particle sizes by the pulverization method, and still smaller particles cannot be obtained.
Recently, the need of higher image quality is increased. In particular, to actualize formation of a color image of high resolution, smaller particle size and more spherical shape are required to achieve. With smaller particle size, it is possible to have higher reproducibility of dots. With more spherical shape, it is possible to achieve improvement in development and transfer properties. Furthermore, in recent days, a cleaning-less system has become widely used in which higher transfer efficiency is achieved using toners with more spherical shape.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2004-177555, a cleaning-less image forming apparatus in which with the use of a spherical toner having at least one of a charge control agent and an organic fine particle present on the surface, higher transfer efficiency is achieved, and thus a residual transfer toner is reduced. In such an image forming apparatus, out of residual transfer toners, only inversely charged toner is collected with a brush roller, emitted to a photoconductor drum at a predetermined timing, and transferred to an intermediate transfer belt. Then, when this inversely charged toner passes through the charged region, charge bias is stopped, or a charging roller is spaced apart from the photoconductor drum, thereby preventing charge defects of an image bearing member due to that the residual transfer toners are adhered to a charging member.
However, as toner has a smaller particle size, transfer efficiency decreases. This is due to the fact that a non-electrostatic adhesion force such as van der Waals force acting between toner and a photoconductor increases with increasing weights of toner particles and thus the toner particles become more difficult to be released from the photoconductor.
To solve these problems, an image forming apparatus described in JP-A No. 2004-177555 is arranged to collect toner without using a cleaning member utilizing a higher transfer property of toner having been spherical-shaped. Nevertheless, when toner has smaller particles, it is difficult to reliably remove toner in the cleaning-less system.
Accordingly, it is necessary to obtain toner that is suitable for cleaning with the use of a cleaning member, as well as that has spherical shaped particles. In a cleaning step for cleaning toner special-shaped and smaller particle-sized from on the image bearing member, as unit for removing toner left on the image bearing member after transfer operation, a blade cleaning system is employed due to the simple structure and superior removable capability. However, a cleaning blade removes toner while sliding on the surface of an image bearing member, so that the edge portion of a cleaning blade is deformed owing to frictional resistance with the image bearing member, and thus a minute space is made between the image bearing member and the cleaning blade. In this space, the smaller toner particles are, the more likely toner is to get in. The more spherical-shaped toner having got in is, the smaller rolling frictional force is. Consequently, a problem exits in that toner begins to roll in the space between the image bearing member and the cleaning blade, and gets through the cleaning blade, leading to cleaning defects.
As a means of solving such problems, the following deformed toner is proposed in Japanese Patent Application Laid-Open (JP-A) No. 08-044111. This toner is obtained by applying an external force to a spherical toner including at least a low softening-point substance and colorant, as well as having a storage elastic modulus G′ of 8.00×103<G′≦1.00×109 dyne/cm2. With this proposal, however, since deformation treatment keeping the spherical shape of toner is not achieved, a transfer property of toner cannot be maintained.
Further, Japanese Patent Application Laid-Open (JP-A) No. 2000-122347 has an object of providing an image forming method in which there are no cleaning defects even if a spherical toner is used. In this image forming method, used is a developer in which (i) spherical carrier, (ii) spherical toner, and (iii) volume average particle diameter is 1 μm to 8 μm; shape factors SF-1>140 and SF-2>130 are met; and 0.5% to 15% additives are contained by volume of a carrier at the start of image forming.
In the case where still smaller particle-sized toner is required, however, the image forming method of regulating the content of additives having specified shape factors as described in the JP-A No. 2000-122347 is insufficient as unit for improving a cleaning property in blade cleaning system, and thus the occurrence of cleaning defects cannot be prevented.
In addition, proposed is the method in which with a shape factor SF-1, being an index of representing degrees of circularity of a toner particle to be used, and a shape factor SF-2, being an index of representing degrees of concavities and convexities of a toner particle, the shape of toner is regulated, and thus the shape of toner is controlled, to improve a cleaning property.
For example, in Japanese Patent Application Laid-Open (JP-A) No. 2004-053916, the following cleaning unit is proposed. This cleaning unit is constructed such that a cleaning blade and a cleaning brush are disposed in the state of in contact, the nearest distance between the contact edge of the cleaning blade in contact with a transfer belt, and a cleaning brush radius with respect to this contact edge is 0.5 mm to 3 mm, and a counter-rotation amount is a distance between the contact edge of the cleaning belt, and the contact point of the cleaning brush with respect to the transfer belt, or more. In this proposed cleaning unit, used it toner which average circularity is 0.90 to 0.99, shape factor SF-1 is 120 to 180, which shape factor SF-2 is 120 to 190, and which Dv/Dn ratio is 1.05 to 1.30, and which surface includes concavo-convex shape.
However, in the case where the surface of toner is concavo-convex-shaped as in the JP-A No. 2004-053916, since the contact frequency between toner concavities and a carrier comes to be decreased, there will be defects of slow charge rise in the beginning, or less charge density.
As procedures of improving such charge properties of toner, for example, in a pamphlet of International Publication No. WO2004/086149, proposed is toner containing at least one kind of inorganic fine particles in an internal part of a toner base particle. In this proposal, inorganic fine particles contained in toner uniformly reside in an internal part of toner, it is possible to stabilize a charge property. Furthermore, since the burial of external additives can be prevented, it is possible to improve a flow property.
However, the image forming method of the invention described in the International Publication No. WO2004/086149, since the toner surface is not deformed unevenly, includes no function to improve a cleaning property.
Moreover, in the fixing step of the image forming method, the releasing property (hereinafter, may be referred to as “offset resistance”) of toner particles with respect to a heating member is required. This offset resistance can be improved by causing a releasing agent to be resided on the toner particle surface. For example, proposed is the method of improving an offset resistance by causing inorganic fine particles to be localized on the surface of toner particles.
In this proposal, however, a fixing lower limit temperature rises, and low-temperature fixing property, that is, energy saving fixing property is insufficient. In the low-temperature fixing system where still lower fixing is required, a problem exits in that fixing inhibition with inorganic fine particles localized on the toner surface occurs, and thus a sufficient fixing temperature range cannot be obtained.